This invention relates to photographic materials and to a process for reducing their sensitivity to ionising radiation.
It is well known that photographic materials, particularly high speed photographic emulsions, are sensitive to ionising radiation. Since ionising radiation is present throughout the environment, contact with the photographic material is unavoidable.
U.S. Pat. No. 4,264,724 describes a process for increasing the sensitivity upon exposure to light of a photographic emulsion by exposing the silver halide emulsion during preparation to an ionizing radiation. The effect is said to produce a silver halide gelatin emulsion having grains of emulsion containing in their interior sub latent image nuclei for providing sensitivity on exposure to light. However one of the effects of ionizing radiation on a photographic material between its preparation and use is premature ageing through high fog. In addition there is a significant increase in granularity that is particularly severe for color negative materials.
U.S. Pat. No. 5,096,804 discloses that silver halide emulsions sensitized by the addition of a gold sensitizing agent and a sulphur sensitizing agent are subject to fog and deterioration in graininess due to natural radiation and discloses that film having a lower total gold/silver weight ratio exhibited less fog and deterioration in graininess than films in which the ratio is higher.
U.S. Pat. No. 5,284,740 discloses that the increase in fog and decrease in sensitivity due to radiation of a silver halide color photographic light sensitive material can be mitigated by including a sulphur containing compound bearing an organic heterocyclic group.
The inventors of the present invention have found that the high granularity is caused by the dispersity of latent image over the grain surface, which results in several independently developable centres so that the grain produces an abnormally large dye cloud that is bad for granularity. This high granularity occurs at low densities near fog where it is particularly noticeable in the final image.
The present invention provides a solution to the problems of graininess and fog due to radiation by the provision of a process for treating photographic materials to reduce their sensitivity to ionising radiation.
According to the present invention there is provided a process for the treatment of a photographic material, particularly a high speed photographic emulsion, which process comprises prior to exposure, subjecting the material, to a treatment that reduces the surface image dispersity so that fewer centres develop on each grain.
Photographic emulsions treated in accordance with the invention have reduced granularity when subjected to ionising radiation.